Field of the Invention
The present invention relates to a liquid ejection head and an apparatus and a method for printing on a printing medium by ejecting ink onto the printing medium.
Description of the Related Art
In ink-jet printing apparatuses, an example of a method for achieving high-speed printing is a method of reducing the number of times of scanning in printing and an example of a method for achieving high image quality is a method of decreasing the size of ink droplets. Examples of a method for achieving the above two methods without changing the size of the print head include a method of increasing the number of ink ejection ports by disposing ink ejection ports at high density and a method of increasing the frequency of ink ejection. However, it is known that printed images are affected by an airflow generated due to splashes of ejected ink droplets and an airflow generated due to the relative motion of the print head and the printing medium.
FIG. 11 is a diagram illustrating cylindrical vortices 12, which are airflows generated between a print head and a printing medium in a conventional ink-jet printing apparatus. As illustrated, airflows generated due to ejection of ink between the print head and the printing medium and airflows generated due to the relative motion of the print head and the printing medium interfere with each other to generate cylindrical vortices 12. Such vortices 12 can affect the landing positions of the ejected ink droplets. In particular, deviation of the landing positions of what is called satellite droplets accompanying main ink droplets and having diameters smaller than those of the main droplets cause streaks and turbulence like wind ripples, as observed on sand dunes, (hereinafter, referred to as wind ripples) to decrease the image quality.
FIG. 12 is a diagram illustrating a method of ink-jet printing disclosed in U.S. Pat. No. 6,997,538 B1.
In the method disclosed in U.S. Pat. No. 6,997,538 B1, in order to eliminate the cylindrical vortices 12 generated by droplets ejected from ink ejection port arrays forward of the moving direction of the print head, gas is introduced between a print head and a printing medium. However, the method disclosed in U.S. Pat. No. 6,997,538 B1 requires that the gas introduced have a sufficient flow rate to generate much more airflows than airflows generated due to the relative motion of the print head and the printing medium. Thus, the airflows caused by the introduced gas can significantly deviate the landing positions of the ejected ink droplets from desired landing positions. This can decrease the image quality.
The inventors found that when the ejection ports of the print head are disposed at high density, or when the ejection frequency is set relatively high, vortices generated between the print head and the printing medium can be unstable because of the unstable performance of the gas. The inventors also found that the unstable vortices can disturb the landing positions of the satellite droplets to generate streaks in the printed image or turbulence like wind ripples, as observed on sand dunes, to decrease the image quality (FIG. 11).
The present invention provides a liquid ejection head and an apparatus and a method of printing in which generation of wind ripples caused by the displacement of ink droplets landing positions is reduced or eliminated, enabling high-quality printing.